Problem 15 - 43 Years JEE ADVANCED (1978-2020) + JEE MAIN Chapterwise & Topicwise Solved Papers Chemistry

Question

The compound which reacts fastest with Lucas reagent at room temperature is (a) butan-1-ol (b) butan-2-ol (c) 2-methylpropan-1-ol (d) 2 -methylpropan-2-ol

Step-by-Step Solution

Answer

2-methylpropan-2-ol reacts fastest with Lucas reagent.

Step 1: Understand Lucas Reagent

Lucas reagent is a solution of anhydrous zinc chloride in concentrated hydrochloric acid. It is used to classify alcohols of low molecular weight and determines whether an alcohol is primary, secondary, or tertiary. The alcohol's reactivity in substitution reactions with Lucas reagent typically increases from primary to tertiary alcohols.

Step 2: Analyze the Reaction Mechanism

The reaction with Lucas reagent proceeds via an S_N1 reaction mechanism for secondary and tertiary alcohols. In this mechanism, the rate-determining step is the formation of a carbocation. Tertiary alcohols form more stable carbocations than secondary or primary ones, making them react faster.

Step 3: Identify the Type of Alcohols

- Butan-1-ol is a primary alcohol. - Butan-2-ol is a secondary alcohol. - 2-methylpropan-1-ol is a primary alcohol. - 2-methylpropan-2-ol is a tertiary alcohol.

Step 4: Determine Reactivity Based on Alcohol Type

According to the S_N1 mechanism, the reactivity order is tertiary > secondary > primary alcohols. Therefore, 2-methylpropan-2-ol (tertiary) should react faster than the other options.

Step 5: Conclude the Fastest Reacting Alcohol

Considering the analysis, 2-methylpropan-2-ol, which is a tertiary alcohol, reacts the fastest with Lucas reagent at room temperature.

Key Concepts

Alcohol Classification

Alcohol classification is a system used to categorize alcohols based on the number of carbon atoms connected to the carbon bearing the hydroxyl (\(\text{-OH}\)) group. This classification helps predict how different alcohols will behave chemically, particularly in reactions like those involving Lucas reagent.

• Primary Alcohols: These alcohols have the hydroxyl group attached to a carbon atom that is connected to only one other carbon atom. An example is butan-1-ol.
• Secondary Alcohols: These alcohols have the hydroxyl group attached to a carbon atom connected to two other carbon atoms. Butan-2-ol is an example.
• Tertiary Alcohols: In these alcohols, the carbon bearing the hydroxyl group is connected to three other carbon atoms, such as in 2-methylpropan-2-ol.

The classification is crucial because it influences the stability of the carbocation formed during certain reactions, affecting the overall reactivity and reaction rates.

Substitution Reactions

Substitution reactions are a fundamental type of chemical reaction where one functional group in a molecule is replaced by another. They are essential in organic chemistry due to their role in forming new compounds. In the case of alcohols reacting with Lucas reagent, the hydroxyl group is replaced by a chlorine atom.
Lucas reagent, which contains zinc chloride in hydrochloric acid, facilitates these substitution reactions by forming the necessary reactive intermediate. The speed and success of these reactions depend heavily on the type of alcohol:

• Primary alcohols react very slowly due to the instability of their resulting carbocations.
• Secondary alcohols react at a moderate pace.
• Tertiary alcohols react quickly because of the relatively stable tertiary carbocations they form.

Understanding substitution reactions helps chemists manipulate and devise reaction pathways strategically to synthesize desired compounds and chemicals.

S_N1 Reaction Mechanism

The S_N1 reaction mechanism (Substitution Nucleophilic Unimolecular) is characterized by a two-step process where reaction kinetics depend on the formation of a carbocation intermediate. This mechanism is typical for reactions involving secondary and tertiary alcohols with Lucas reagent.

• Step 1: Carbocation Formation: The first step involves the dissociation of the leaving group, such as the hydroxyl group, to form a carbocation. This is the rate-determining step, which influences the reaction speed.
• Step 2: Nucleophilic Attack: In the subsequent step, a nucleophile, often chloride ion from the hydrochloric acid in Lucas reagent, attacks the carbocation, completing the substitution.

In the context of Lucas reagent, tertiary alcohols are highly reactive due to their ability to stabilize carbocations effectively, making them react quickly in S_N1 reactions. This is why compounds like 2-methylpropan-2-ol react the fastest among the presented options.